The present invention relates to an exposure device which is applied to a projection mechanism of a copier, and especially to an exposure device having a light shading member to correct light distribution .
In the projection mechanism of a copier, a lens is essential to form a projected image into an image on a photoreceptor, which is an image carrier. The lens has the following characteristics: luminous flux density on an image point located outside of an optical axis is reduced in proportion to cos.sup.4 .theta. of the luminous flux density of the image point located on the optical axis, which is the "cos.sup.4 .theta. law". Accordingly, in a conventional copier which can magnify a document image, light distribution is adjusted when a side of a slit, which is an optical path regulation member, is provided in the manner that the slit can advance or retreat with respect to the optical path; or when a light shading member, which is different from the slit, is provided in the manner that it can move perpendicularly to the optical axis.
Further, in order to properly correct a light amount with respect to all the projection magnification ratios, the following exposure device has been proposed, for example, as disclosed in Japanese Patent Publication Open to Public Inspection No 63-106639: the exposure device has; a light shading member which is rotated around the shaft, which is provided in the direction perpendicular to the optical axis and provided in the longitudinal direction of a cross section of the optical path with respect to the optical path regulated in the form of a slit by an optical path regulation member; and a light shading member rotating mechanism which corrects the distribution of the light amount on the photoreceptor surface of an image light passing through the lens, corresponding to the magnification ratio of the lens while the light shading member is being rotated.
In the conventional exposure device, a light source having the distribution characteristics of light distribution in which the distribution of the light amount on the photoreceptor surface is uniform at a predetermined magnification ratio when no shading is conducted. As a light shading member, a light shading plate is provided, the thickness of which is thin, and in which the length in the cross direction of the cross section of the optical path is not uniformly changed in the adjoining positions when the plate is rotated.
A predetermined magnification ratio of the lens can be arbitrarily determined like, for example, the maximum magnification ratio, or minimum magnification ratio, and further, an intermediate magnification ratio having the value of 1. However, in any case, the light source should have light distribution characteristics in which the light amount distribution on the photoreceptor surface becomes uniform at the magnification ratio when shading is not conducted. When the predetermined magnification ratio is the maximum magnification ratio, the light amount of the peripheral portion apart from the optical axis is most conspicuously lowered. Accordingly, in order to make the light amount distribution uniform, it is necessary that the light amount of the peripheral portion of the light source is greatly increased. When the predetermined magnification ratio is the minimum magnification ratio, the light amount of the peripheral portion is decreased by the least amount, so that the light source has a flat distribution characteristic of light distribution. However, the light amount of the peripheral portion is gradually lowered as the magnification ratio is increased, and the light amount in the center of the optical axis is gradually increased compared with the peripheral light amount. Accordingly, the light shading plate is used so that the light amount of the central portion, which is relatively increased as the magnification is increased, is decreased. Further, when the predetermined magnification ratio is the intermediate magnification ratio having the value of 1, a rotational light shading member in which the light shading plates are combined is provided, and thereby, variation of the entire light amount is decreased through all the magnification range. However, since it is necessary that a shaded light amount of the central portion is equal to that of the peripheral portion at the life-size magnification ratio, a projected area of the light shading plate at that time is maximum, and lowering of the entire light amount is maximum.
However, in the above-described conventional exposure device, when, for example, the distribution characteristic of the light distribution of the light source is set to that of the maximum magnification ratio, it is necessary that an emitted light amount of the peripheral portion is greatly larger than that of the optical axis portion. When the distribution characteristic of the light distribution of the light source is set to that of the minimum magnification, which is comparatively flat, and the light amount of all portions is appropriate at the minimum magnification ratio, it is necessary to shade the light amount of the central portion which is relatively increased as the magnification ratio is increased, and therefore, the light amount of all portions is insufficient, which is disadvantageous. Further, when the distribution characteristic of the light distribution of the light source is set to that of the life-size magnification ratio, the shaded light amount is maximum at the life-size magnification ratio, which is most frequently used, and when the light amount is set to the foregoing, the light amount other than during life-size magnification is excessive, which is disadvantageous.